The present invention relates to an improved cistern specifically designed for holding and carrying liquids.
As known, liquid holding and carrying cisterns must meet a plurality of rather strict requirements, in order to be certified as suitable for public use.
The above requirements, in particular, are very stringent for those cisterns specifically designed for carrying liquids in general, such as food and industrial liquids, because of a great variation of the specific gravity thereof, affecting the stress the holding and carrying cistern is subjected to.
These stress values, in fact, vary from 0.8-0.9 to less than 2 Kg/m.sup.3, and, since the cisterns of the above mentioned type have a volume of several cubic meters, weight variations of the order of tons will occur.
Prior cisterns for the intended applications conventionally comprise a bottom pallet, thereon is fastened a metal outer cage, encompassing a holding tank usually made of a plastic material and fastened to the top of the cage, for example by two or more metal cross-members.
While the above mentioned prior cisterns solve the indicated technical problem, they, however, are affected by the following disadvantages.
The metal pipes forming said outer cage have contours which are not suitable for mating the operating conditions of automatized lines. In particular, great problems are encountered for automatizing the welding operations provided for firmly fastening the cage forming pipes at crossing points thereof.
Actually, in the prior art, the mentioned cage forming pipes are welded by manually controlled welding apparatus, thereby greatly increasing the welding time and cost, because of a substantial requirement of manual labour.
Moreover, the quality controls too are performed by manually controlled implements, thereby they will require a lot of labour, and, moreover, being scarcely reliable.
In this connection it should be pointed out that a critical aspect of the above mentioned quality controls is that related to the mentioned welding operations proper, since the welded regions must overcome very rigorous and accurate control tests; thus, in prior cistern making methods, the above mentioned quality controls will require a very long time with a consequent negative effect on the cistern making cost.
The document EP-A-0 755 863 discloses a cistern substantially according to the preamble of claim 1.